Following a voltage depression in a power delivery system caused by a fault and/or outage of a major electric power transmission device such as a transmission line, a large power transformer, or a power plant, power system loads (e.g. induction motor loads) will draw reactive power from the power delivery system in an attempt to recover. This additional reactive power demand will further suppress voltage unless some other source of reactive power is introduced into the system. If voltage suppression lasts long enough, the net reactive power deficit in the system can lead to voltage collapse.
In order to restore voltage under these conditions, reactive power is temporarily supplied to the system that exceeds the normal or rated reactive power demand. However, taking corrective action by providing excess reactive power may also lead to an overvoltage condition if the excess reactive power is supplied to the power delivery system beyond the moment of voltage recovery. Overvoltage conditions will stress system insulation, which can lead to flashovers, equipment damage (e.g., transformers, lightning arresters, etc.), and/or additional system faults.
Traditional solutions to this problem include static VAR compensators (SVC), static compensators using voltage-source converters (STATCOM), and synchronous condensers. However, such devices can be relatively expensive and rely on the active control of high-power semiconductor technology or the inherent transient reactive power capabilities of synchronous machines. Switched shunt capacitor banks have also been used, relying on active controls including voltage measurement and switch timing to open and close breakers to provide reactive power to the system. However, if the reactive power is excessive, the opening of the circuit breakers may not occur fast enough to be effective to control the bus voltage within its normal operating range and a temporary overvoltage may occur.
Therefore, a need exists for a relatively low cost and effective system and method for providing dynamic reactive support to a power transmission system.